One S IMO.
It is just the X that isn't optimized well enough with sense and tegra (this should in theory change as the nexus 7 is also using the tegra chipset and the nexus 7 is said to run faster and smoother than the GS 3

as long as HTC don't **** up

the X should be the same when it receives JB), the S is optimized perfectly well as the performance is pretty much on par with the GS 3 and battery life is slightly better (I know the res./screen size isn't the same, but it has a smaller battery as well, just a more PE chip)
The GS 3 only appears smoother than the S/X due to the animations. If you were to flash the exact same animations of the GS 3 on the S and set the same speed etc. it is guaranteed that you will barely notice any difference especially once JB arrives.
Also looking at the review of the GS 3 S4 CPU version with the same GPU as well although it has 2GB RAM, the XL and S (to be expected due to the smaller res. on the S though) are quite a bit better than the GS 3 S4 version in performance and battery life benchmarks, so sense must be pretty well optimized.
http://www.anandtech.com/show/6022/samsung-galaxy-s-iii-review-att-and-tmobile-usa-variants/4
Either that or touchwiz is just poorly optimized with the S4 and adreno 225
And even then the GS 3 quad version here doesn't score much better than the S/XL in quite a few tests, rapes them in GPU dependant tests though as the GPU is far better.
In real world usage there isn't a massive difference especially once you get a 3rd party launcher on.
Pretty much exactly what Robbo said in his guide there Mike, although IMO the camera is a tie with the GS 3 as the features/UI etc. are far better than touchwiz alternative and the quality of photos seem to be better in some shots and worse in other shots going by Stoosh's comparison shots in the S thread. Also comes down to what you prefer, more vibrant colours or more natural colours, IMO the vibrant photos are more "pleasing" to look at.
The 1080P video recording on the GS 3 is much better though, except the stereo recording and the S has slightly better video recording than the X, no idea why though.
Also think it is a tie for phone/call performance, the S is great for call quality and performance regarding that, it is only a problem if you encounter the signal issue, some people have it, others don't.
Basically go for the X for these reasons:
- storage
- NFC
- better/larger screen although it is IPS, so you loose the blacks and more vibrant colours
- better developing community
Go for the S for these reasons:
- smaller/more comfortable to hold
- better audio
- faster/smoother
- better battery life
- better perceived and build quality
Software is exactly the same.
EDIT:
However, you can eliminate some of the main areas of touchwiz:
I recommend these apps:
browser - quick ics browser
launcher - nova or apex
contacts/dialer - ex dialer with black ICS theme
SMS - go sms
calendar - android 4 calendar
music - poweramp or winamp
gallery - gallery ICS
weather widget - snowstorm, fancy widgets, weather eye
calendar/agenda widgets - APW widgets with google ICS theme, smooth calendar, simple calendar
keyboard - swiftkey 3 or stock ICS
lockscreen - widget locker
The rest such as system menus and integrated features you can't do anything about unless you root and flash a custom/stock ROM but it will be no replacement for HTC sense if you're a massive fan of it.